Granular pumice and method for producing granular pumice

ABSTRACT

The invention relates to granular pumice, wherein the surface is covered with a hydrophobic coating.

The present application claims priority from PCT Patent Application No.PCT/EP2011/053287 filed on Mar. 4, 2011, which claims priority fromGerman Patent Application No. DE 10 2010 021 532.5 filed on Mar. 26,2010, the disclosures of which are incorporated herein by reference intheir entirety.

1. FIELD OF THE INVENTION

The invention relates to granular pumice, wherein the surface is coveredwith a hydrophobic coating.

It is noted that citation or identification of any document in thisapplication is not an admission that such document is available as priorart to the present invention.

Granular pumice is used primarily as lightweight aggregate for a varietyof building products and building chemical products. Mention may be madeof plasters and renders and mortars, fine concretes, exterior wallcladding boards and lightweight boards and also fire and noiseprotection boards, insulation products and the like. However, granularpumice is also used as lightweight aggregate for plastics. Theadvantages of granular pumice are its very low weight, the highcompressive strength and also very good thermal insulation. It is alsowell suited for sound absorption, displays a high breathing activity, isnot combustible and is solvent-free and has a neutral odor, and also is100% mineral and recyclable.

However, as a mineral, porous natural product, pumice displays a certainwater absorption capability. As a result, when pumice is used aslightweight aggregate wherever water is used for mixing, somewhat morewater has to be added compared to a lightweight aggregate having nowater absorption capability or a significantly lower water absorptioncapability, for example the siliceous foams which are likewisefrequently used. This results in the productivity of the mixedcomposition containing the added pumice being somewhat lower than theproductivity of a composition having, for example, siliceous foam aslightweight aggregate.

It is noted that in this disclosure and particularly in the claimsand/or paragraphs, terms such as “comprises”, “comprised”, “comprising”and the like can have the meaning attributed to it in U.S. patent law;e.g., they can mean “includes”, “included”, “including”, and the like;and that terms such as “consisting essentially of” and “consistsessentially of” have the meaning ascribed to them in U.S. patent law,e.g., they allow for elements not explicitly recited, but excludeelements that are found in the prior art or that affect a basic or novelcharacteristic of the invention.

It is further noted that the invention does not intend to encompasswithin the scope of the invention any previously disclosed product,process of making the product or method of using the product, whichmeets the written description and enablement requirements of the USPTO(35 U.S.C. 112, first paragraph) or the EPO (Article 83 of the EPC),such that applicant(s) reserve the right to disclaim, and herebydisclose a disclaimer of, any previously described product, method ofmaking the product, or process of using the product.

SUMMARY OF THE INVENTION

The invention therefore addresses the problem of providing a granularpumice whose properties are improved, especially for use as lightweightaggregate to produce a composition to be made up using a liquid.

To solve this problem, the invention provides for the surface of thegranular pumice to have a hydrophobic coating.

DETAILED DESCRIPTION OF EMBODIMENTS

It is to be understood that the figures and descriptions of the presentinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the present invention, while eliminating,for purposes of clarity, many other elements which are conventional inthis art. Those of ordinary skill in the art will recognize that otherelements are desirable for implementing the present invention. However,because such elements are well known in the art, and because they do notfacilitate a better understanding of the present invention, a discussionof such elements is not provided herein.

The present invention will now be described in detail on the basis ofexemplary embodiments.

The granular pumice according to the invention, which as a naturalmaterial has a certain water absorption capability, is, according to theinvention, coated with a hydrophobic surface coating and is thushydrophobicized. This surface layer applied by means of appropriatetreatment of the pumice results in the pumice granules being completelyhydrophobic and retaining their hydrophobic property, as long-termexperiments have shown. The hydrophobic surface coating consequentlygives the granular pumice of the invention a property which haspreviously not been ensured and has a decisive effect on the range ofuses of the pumice or on the properties of the products to which thepumice of the invention is added. Firstly, the hydrophobicized pumice ofthe invention can be used in a variety of liquid systems or systemscontaining a fluid such as make-up water and the like or a solvent orthe like. Mention may be made of tile adhesives or board adhesives,lightweight plasters and renders, reinforcing trowel-applied mortars andalso varnishes, paints and paste-like systems. Many of these systems arenot mixed up only on site but have already been ready-mixed and packagedat the factory, so that it has to be ensured that they retain theirfluid or paste-like properties unchanged for a long time. This canadvantageously be ensured by use of the granular hydrophobicized pumiceof the invention, in contrast to the use of siliceous foam which, asdescribed, has an albeit low water absorption capability. In addition,the fact that the pumice of the invention no longer has any waterabsorption capability allows more precise setting of the water/cementvalue when using the pumice as lightweight aggregate for mortar andsimilar compositions. Furthermore, the apparent particle density of thepumice of the invention can be significantly reduced compared to purepumice, from about 0.9 kg/dm³ for unhydrophobicized pumice to about 0.5kg/dm³ for hydrophobicized pumice according to the invention. This leadsto an increased productivity and lower dry bulk density of, for example,the mortar or tile adhesive which is admixed with the pumice accordingto the invention. The hydrophobic property of the pumice also enables animproved distribution of this in cement-bonded systems to be achieved,which makes improved, because homogeneous, hydrophobicization of the endproduct possible. This means that the water absorption capability of theend product is lower compared to other lightweight aggregates.

Overall, the granular hydrophobicized pumice of the invention offers anessential widening of the property spectrum and thus also the usespectrum.

The pumice can, in an embodiment of the invention, be coated with asilane and/or siloxane layer which is hydrophobic. Such a silane orsiloxane layer can be applied using various silanes or siloxanes, eithercyclic or linear silanes or siloxanes, with, in particular, linear orbranched silanes or mixtures thereof or siloxanes which are virtuallyinsoluble in water being suitable. These include, for example, siliconeresins or silicone resin solutions, oligomeric siloxanes and polymericsiloxanes. The use of silicones is also possible. In general, any typeof alkylpolysiloxanes and/or alkylalkoxysilanes and also siloxanemixtures of reactive organosiloxanes and polydimethylsiloxanes can beused as hydrophobic coating for the pumice surface. Among these,preference is given to using a mixture of reactive organosiloxanecompounds and polydimethylsiloxanes, particularly preferably present asemulsion, for forming the coating.

The pumice can also, additionally or alternatively, be coated with acoating of at least one organic fatty acid or at least one salt of anorganic fatty acid. Such acids and salts include, for example, stearicacid and various salts of stearic acid, i.e. stearates (e.g. Na, K, Ca,Al, Zn, Zr or NH₄ stearate or triethanolammonium stearate), oleic acidand various salts of oleic acid, i.e. oleates (e.g. Na and NH₄ oleate)and also tall oil fatty acid and salts thereof and resin acids and saltsof resin acids.

In addition or alternatively, the coating of the pumice surface can alsoconsist of a mineral oil or a mineral oil emulsion. Mineral oils consistessentially of a mixture of paraffinic, naphthenic and aromaticconstituents and also alkenes.

It is also possible, in addition or alternatively, for the coating ofthe pumice surface to consist of at least one alkane. These also includewaxes and wax oil emulsions, optionally on a paraffin basis. PE/HDPEwaxes, microwaxes, Fischer-Tropsch waxes, montan waxes, ester waxes,amide waxes, natural waxes, carnauba waxes, PTFE waxes, wax compounds,polymer-wax compounds or PP waxes are also conceivable.

In addition or alternatively, the pumice surface can also have a coatingconsisting of bitumen or a bituminous emulsion. This likewise has astrongly water-repellent or hydrophobic behavior.

Apart from the pumice of the invention itself, the invention provides aprocess for producing pumice of the type described. The process of theinvention is characterized in that the granular pumice is wetted with anemulsion containing at least one hydrophobicizing agent which forms theinner phase dispersed in the outer phase and is subsequently dried, withthe hydrophobicizing agent forming a hydrophobic coating on the surfaceof the pumice granules. The hydrophobicizing agent is preferably used asemulsion.

Although it is possible to wet the pumice by dipping in the emulsion,i.e. to pour the pumice into a sufficiently large amount of emulsion,stir it there and allow the emulsion or the hydrophobicizing agent tosoak in or become attached and, after a sufficient residence time, takeout the pumice, allow it to drip and dry it, an advantageous embodimentof the invention provides for spraying the hydrophobicizing emulsion ora dilution thereof onto the pumice. Spraying-on is advantageouslycarried out while the pumice is being mixed. That is to say, the pumiceis mixed continuously, for example in a plowshare mixer, while theemulsion is sprayed on continuously or intermittently by means of asuitable spraying device so that the emulsion is mixed in as a result ofthe continuous mixing and thus becomes finely and homogeneouslydistributed on the surface of the pumice granules. When the emulsionmatched to the amount of pumice to be treated and containing asufficient amount, likewise matched to the amount of pumice to betreated, of added hydrophobicizing agent has all been sprayed on andmixed in, mixing can be continued to achieve further homogenization ofthe emulsion distribution. At the end of mixing, the pumice is dried inorder to drive off the outer phase of the emulsion, i.e. evaporate thewater, so that the water-free hydrophobicizing layer is formed.

The basic mixing time should be in the range from 1 minute to 25minutes, in particular from 10 minutes to 15 minutes, with the emulsionbeing sprayed on during only part of the total mixing time. For example,spraying-on can be carried out during the first 5 minutes of a totalmixing time of 20 minutes and mixing can subsequently be continued for afurther 15 minutes in order to homogenize the emulsion orhydrophobicizing agent distribution.

The heat treatment, i.e. the drying of the pumice, is advantageouslycarried out at a temperature of 0-200° C., preferably 130°-180° C., sothat it is ensured that the water which is generally used as outer phaseis driven off as quickly as possible. A preferred drying temperature isabout 150° C.

As hydrophobicizing agent, preference is given, according to theinvention, to using at least one silane and/or at least one siloxane,with the above-described both cyclic and linear or branched silanes andmixtures thereof being used. Linear silanes or siloxanes in particulardisplay no solubility in water.

In addition, it is possible to use one or more of the abovementionedsilane- and/or siloxane-based materials.

According to the invention, it is possible to use silanes, siloxanes andany mixture thereof with use being made of

A1) silane of the general formula

-   -   R³—Si—(OR²)₃    -   where    -   R² is a methyl or ethyl radical and    -   R³ is an alkyl radical having from 3 to 12 carbon atoms,    -   or an aminoalkyl radical having from 1 to 6 carbon atoms or    -   a radical of the general formula H₂N—(CH₂)_(x)R⁴—(CH₂)_(y)—,        where    -   R⁴ is an oxygen, sulfur, —NH—    -   or    -   —NH—CH₂—CH₂—NH— radical and    -   x≧2 and    -   y≧2, or    -   a radical of the general formula    -   (R²O)₃Si—(CH₂)_(x)—NH—(CH₂)_(x)—,    -   R² and x are as defined above,

-   -   and n is an integer from 1 to 5,    -   or        A2) a polysiloxane or siloxanol of the general formula

-   -   where    -   R⁵ is a methyl or phenyl radical but at least 90% of the        radicals R⁵ are methyl radicals    -   and m=20 to 250,    -   where a mixture consists of A1) and A2),

where the ratio of A1) to A2) is kept so that one SiOH group of thecompound A2) corresponds to from >1 to 30R² groups of the compound A1)and R² is an alkyl radical having from 1 to 4 carbon atoms. Examples ofthe radical R² are the methyl, ethyl, propyl and butyl radicals.Preference is given to the methyl and ethyl radicals. Constituent A1) isan alkoxysilane of the general formula R³—Si—(OR²)₃. While the radicalR² has the meaning indicated above, the radical R³ is characterized inthat it has a reactive group and is bound via carbon to the siliconatom. R³ can have the following meanings:

(1) R³=aminoalkyl radical whose alkyl radical has from 1 to 6 carbonatoms. Examples of such radicals are the ethyl, propyl, butyl andhexylamine radicals, with preference being given to the radicals

—(CH₂)₃—NH₂ and

(2) R³=radical of the general formula H₂H—(CH₂)_(x)R⁴—(CH₂)_(y)—, whereR⁴ is an oxygen, sulfur, —NH— or —NH—CH₂—CH₂—NH— radical and x≧2 andy≧2. Examples of such radicals are H₂N—(CH₂)₃— andH₂N—(CH₂)₂—NH—(CH₂)₃—.

(3) (R²O)₃Si—(CH₂)_(x)—NH—(CH₂)_(x)—. Examples of such radicals are(C₂H₅O)₃Si—(CH₂)₃—NH—(CH₂)₃— and (CH₃)₃Si—(CH₂)₂—NH—(CH₂)₂—.

R³=radical of the general formula

and n is an integer from 1 to 5. An example of a particularly preferredepoxide is

Radicals having the meaning —(CH₂)₃—NH₂, —(CH₂)₂—NH—(CH₂)₃—NH₂ or (4)are particularly preferred as radicals R³.

Constituent A2) is an α,ω-siloxanol of the general formula

-   -   where R⁵ is a methyl or phenyl radical. At least 90% of the        radicals R⁵ have to be methyl radicals. Particular preference is        given to α,ω-dimethylsiloxanols. m is from 20 to 250 and is        preferably from 30 to 80.

The ratio of the constituents A1) and A2) has to be set so that one SiOHgroup of the compound A2) corresponds to from >1 to 3 OR² groups of thecompound A1). The ratio of OH:OR² is preferably 1:2.

The silanes A1 and/or siloxanols A2 can be used either alone or in anymixtures with one another.

The mixtures can, as solutions, contain solvents such as alcohols, inparticular in the absence of water, or in the presence of water be inthe form of emulsions.

These products can have an active content of 0.1-100% in water asemulsion or in solvents such as alcohols, hydrocarbons as solution.

These products can have an active content of from 0.1 to 99.9% by weightof silanes A1 and/or of siloxanols A2 dissolved or emulsified in wateror dissolved in solvents such as alcohols or hydrocarbons.

A modification of the preparation according to the present invention ischaracterized in that the components A1) and A2) are not presentindividually but as their reaction product which can be obtained bycompletely or partly reacting the components A1) and A2) with oneanother taking into account the stoichiometric ratios by heating to from150 to 200° C. before emulsification of these components. It has beenfound that when such a modified preparation according to the inventionis used, good results in respect of the water-repellent effect arelikewise achieved.

As hydrophobicizing agent, it is optionally possible to use at least oneorganic fatty acid or at least one salt of an organic fatty acid eitheradditionally or alternatively. Here, reference is once again made to theorganic fatty acids mentioned by way of example above or salts of theorganic fatty acids.

The use of a mineral oil as hydrophobicizing agent is also conceivable,either as an alternative or in addition.

The same applies to the use of an alkane as hydrophobicizing agent.

The use of bitumen as hydrophobicizing agent is also conceivable, eitherin addition or as an alternative.

The content of hydrophobicizing agents, measured in g, in the emulsionshould according to the invention be 0.1%-2.5%, in particular 0.3%-1%,of the amount of pumice to be wetted in g. This means that, tohydrophobicize 1 kg of pumice, with a weighed-out amount of, forexample, 0.5% of hydrophobicizing agent, the emulsion contains an amountof 5 g, for example, of hydrophobicizing agent. The amount of outerphase used, i.e., for example, water, is advantageously also determinedby the amount of pumice to be wetted. Thus, the amount of outer phase,in particular water, measured in g, should be ⅕−⅓, in particular ¼, ofthe amount of pumice to be wetted, likewise measured in g. Based on theexample of 1 kg of pumice, 250 g, for example, of water (=250 ml ofwater) in which, for example, the abovementioned 5 g of hydrophobicizingagent are emulsified should be used.

In various studies and comparative tests, the properties of the pumiceaccording to the invention have been checked and compared with variouscomparative materials.

A particular amount of pumice according to the invention was firstlyproduced. Pumice having the trade name “ROTOCELL” from ROTEC GmbH & Co.KG and having a particle size of 0.09-0.3 mm and a bulk density of 390kg/m³+/−15% was used as starting material. 400 g of this pumice wereweighed out dry.

100 ml of water (=100 g {circumflex over (=)} 25% of the mass of pumice)were then measured out to produce an emulsion. 2 g of hydrophobicizingagent, namely a reactive organosiloxane/PDMS mixture which is obtainableunder the trade name “Sitren 595” from Evonik Goldschmidt GmbH (2g{circumflex over (=)}0.5% of the mass of pumice), were added to thewater. The water and the hydrophobicizing agent were mixed by shakingand the hydrophobicizing agent was thus finely emulsified. The amount ofwater can naturally be varied. The important thing is that the contentof hydrophobicizing agent added is always in the required ratio to themass of pumice to be wetted, and is also all added to the pumice. Themore emulsion in which the amount of siloxane based on the mass ofpumice is emulsified that is produced, the more emulsion also has to bemixed with the pumice.

In the next step, the pumice was introduced into a mixer which mixes thepumice by means of a kneading hook. On commencement of mixing, theemulsion was sprayed on using a spray bottle while mixing was carriedout continuously. After the spraying-on operation was complete, afterabout 5 minutes, stirring was continued for another 15 minutes.

When stirring was complete, the wetted pumice was taken out and placedin a dish in a drying oven at 150° C. for drying. The dried pumice wassubsequently taken out and cooled in the air of the room.

In a long-term test, the water uptake capability of the pumice accordingto the invention was firstly examined.

For this purpose, an amount of exactly 500 ml of distilled water wasplaced in each of two upright cylinders having a reading-off volume of1000 ml. Exactly 100 g of pumice, obtainable under the trade name“ROTOCELL” and having a particle size of 0.09-0.3 mm, which had not beenhydrophobicized according to the invention, were placed in the oneupright cylinder. Exactly 100 g of the hydrophobicized “ROTOCELL” pumiceproduced as described above were placed in the other upright cylinder.

The unhydrophobicized pumice sank immediately after addition. Afterescape of the air from the pore volume of the pumice, a reading-offvolume of 620 ml was determined.

In contrast, the pumice which had been hydrophobicized according to theinvention did not sink. A reading-off volume of 720 ml was determined.

After introduction of the pumice and determination of the reading-offvolumes, the upright cylinders were closed by means of films so as to beairtight and left to stand in the absence of vibrations.

The samples were allowed to stand for a week, after which thereading-off volumes were determined again. The same reading-off volumesas in the first reading immediately after introduction of the pumicewere found, i.e. in the case of the upright cylinder containing theunhydrophobicized pumice, a volume of 620 ml; in the case of thecylinder containing the pumice which had been hydrophobicized accordingto the invention, a volume of 720 ml. Thus, no changes have occurred.

It can be seen from this that the pumice which has been hydrophobicizedaccording to the invention does not absorb any water, i.e. it iscompletely hydrophobic.

In a further experiment, three different test specimens of which onecontained the inventive pumice which had been produced as describedabove while the other two contained other aggregates were produced, withproperties of the initially produced specimen composition and alsoproperties of the finished specimens being examined.

A commercial tile adhesive (adhesive mortar) based essentially on cement(portland cement), finely particulate aggregates (ground limestone,silica sand) and additives (methylcellulose, dispersion powder,cellulose fibers, high-alumina cement) was used as starting material. Ofthis three amounts of mortar, each of exactly 600 g, were weighed out.

150 g of a commercial siliceous foam (lightweight foam) having aparticle size of 0.1-0.3 mm were added to the first amount. 150 g of the“ROTOCELL” pumice having a particle size of 0.09-0.3 mm but nothydrophobicized were added to the second amount of mortar. 150 g of theabove-described “ROTOCELL” pumice according to the invention,hydrophobicized, were added to the third amount of mortar.

Each sample was subsequently mixed with water. 345 g of water were addedto the first sample, 375 g to the second sample and 345 g of water tothe third sample.

The productivity of the respective samples was then firstly measured.The productivity of the first sample containing the siliceous foam was870 ml. The sample containing the untreated pumice displayed aproductivity of 815 ml. The sample containing the pumice according tothe invention displayed a productivity of 900 ml. This means that thepumice according to the invention led to a mortar composition having thegreatest productivity, which can be attributed to the hydrophobicproperty of the pumice and, resulting therefrom, its displacement ofwater and its homogeneous distribution in the composition.

The fresh overall densities of the made-up samples were subsequentlymeasured. This was 127 kg/dm³ in the case of the sample containing thesiliceous foam, 127 kg/dm³ in the case of the second sample containingthe untreated pumice and 122 kg/dm³ in the case of the sample containingthe pumice according to the invention. This means that the samplecontaining the pumice according to the invention had a lower density.

In the next step based on the trial for tile adhesives in accordancewith DIN EN12002, a test specimen in the form of a mortar prism wasproduced from each composition in order to examine the deformationbehavior of the mortar. The respective prism had a length of 160 mm, awidth of 80 mm and a height of 80 mm. The prisms produced in each casewere dried for a number of days and subsequently measured.

The individual measurements are shown together with the respectiveamounts of constituents used in the composition in the following table.

Sample: I II III Lightweight Siliceous Unhydro- Hydro- aggregate: foam,particle phobicized phobicized size: 0.1-0.3 mm pumice, particle pumice,particle size: 0.09-0.3 mm size: 0.09-0.3 mm Adhesive 600 600 600 mortar(g): Mass of 150 150 150 aggregate (g) Amount of 345 375 345 water (g)Productivity 870 850 900 (ml) Fresh overall 1.27 1.27 1.22 density ofmortar (kg/dm³): Test specimen: 160 × 80 × 80 160 × 80 × 80 160 × 80 ×80 Mortar prism L × W × H (mm): Dry weight (g) 229 227 223 Dry overall0.224 0.221 0.218 density (kg/dm³): Fracture 0.487 0.408 0.449 load inbending (kN): Bending 0.143 0.12 0.132 tensile strength (N/mm²):Fracture 4.39 3.56 4.52 load in compression (kN): Compressive 0.7 0.60.71 strength (N/mm²):

Firstly, the dry weight of the specimens was measured. This was 229 g inthe case of the test specimen containing the siliceous foam, 227 g inthe case of the test specimen containing the untreated pumice and 223 gin the case of the test specimen containing pumice according to theinvention. This means that the test specimen containing the pumiceaccording to the invention was the lightest of all the test specimens.

Furthermore, the dry overall density was measured. This was 0.224 kg/dm³in the case of the test specimen containing the siliceous foam, 0.221kg/dm³ in the case of the test specimen containing the untreated pumiceand 0.218 kg/dm³ in the case of the test specimen containing the pumiceaccording to the invention. The test specimen containing the pumiceaccording to the invention thus displayed the lowest dry overalldensity.

The fracture load in bending, the bending tensile strength, the fractureload in compression and the compressive strength were then measured. Thedetailed measurements may be found in the table.

The fracture load of the specimen containing the siliceous foam was0.487 kN. While the fracture load of the specimen containing theuntreated pumice was, at 0.408 kN, significantly below the value for thespecimen containing siliceous lightweight foam, the specimen comprisingthe pumice according to the invention had a measured value of 0.449 kN,approximately in the region of the specimen containing siliceouslightweight foam.

The bending tensile strength of the siliceous foam specimen was 0.143N/mm². Here too, the specimen comprising the untreated pumice was, at0.12 N/mm², significantly lower, while the specimen comprising thepumice according to the invention gave a value of 0.132 N/mm², onceagain approximately in the region of the siliceous foam specimen.

In the case of the fracture load in compression, the specimen containingthe pumice according to the invention displays the highest measuredvalue at 4.52 kN, while the specimen containing the siliceous foam had avalue of 4.39 kN and the specimen comprising the untreated pumice had avalue of 3.56 kN. In the case of the compressive strength, too, thespecimen comprising the pumice according to the invention had, at 0.71N/mm², the highest value, compared to 0.7 N/mm² in the case of thesiliceous foam specimen and 0.6 N/mm² in the case of the specimencomprising the untreated pumice

The comparative experiment shows that use of the pumice according to theinvention makes it possible to mix, for example, a tile adhesive whichfirstly has a very good productivity and secondly, as the various testspecimens showed, has very good mechanical properties compared to, inparticular, the mortar/test specimen containing the siliceous foam.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, the preferred embodiments of the invention as setforth above are intended to be illustrative, not limiting. Variouschanges may be made without departing from the spirit and scope of theinventions as defined in the following claims.

1. A granular pumice comprising: a surface coated with a hydrophobiccoating.
 2. The granular pumice as claimed in claim 1; wherein thecoating comprises a silane, a siloxane, or a mixture thereof.
 3. Thegranular pumice as claimed in claim 1; wherein the silane, if utilizedas the coating, is: A1) silane of the general formula: R³—Si—(OR²)₃;where: R² is a methyl or ethyl radical; and R³ is: an alkyl radicalhaving from 3 to 12 carbon atoms; or an aminoalkyl radical having from 1to 6 carbon atoms; or a radical of the general formulaH₂N—(CH₂)_(x)R⁴—(CH₂)_(y)—, where;  R⁴ is an oxygen, sulfur, —NH—, or —NH—CH₂—CH₂—NH— radical;  x≧2; and  y≧2; or a radical of the generalformula:(R²O)₃Si—(CH₂)_(x)—NH—(CH₂)_(x)—,  where R² and x are as defined above;

 where n is an integer from 1 to 5, or wherein the siloxane orsiloxanol, if utilized as the coating, is: A2) a polysiloxane orsiloxanol of the general formula;

where: R⁵ is a methyl or phenyl radical, with at least 90% of theradicals R⁵ being methyl radicals; and m=20 to 250; and wherein themixture, if utilized as the coating, comprises: the silane compound asper A1); and the siloxane or siloxanol compound as per A2); where aratio of A1) to A2) is kept so that one SiOH group of the compound A2)corresponds to from >1 to 3 OR² groups of the compound A1); and where R²is an alkyl radical having from 1 to 4 carbon atoms.
 4. The granularpumice as claimed in claim 1: wherein the coating comprises at least onecompound selected from organic fatty acids and salts of organic fattyacids.
 5. The granular pumice as claimed in claim 1: wherein the coatingcomprises a mineral oil or a mineral oil emulsion.
 6. The granularpumice as claimed in claim 1: wherein the coating comprises at least onealkane.
 7. The granular pumice as claimed in claim 1: wherein thecoating comprises bitumen or a bituminous emulsion.
 8. A process forproducing the granular pumice as claimed in claim 1, comprising wettingthe granular pumice with an emulsion containing at least onehydrophobicizing agent which is distributed in an outer phase of theemulsion, and which forms the inner phase of the emulsion; andsubsequently drying the granular pumice, so that the hydrophobicizingagent forms a hydrophobic coating on a surface of the pumice granules.9. The process as claimed in claim 8; wherein the emulsion is sprayedon.
 10. The process as claimed in claim 8; wherein the granular pumiceis mixed during and after addition of the emulsion.
 11. The process asclaimed in claim 10; wherein the mixing time is 1 min to 25 min.
 12. Theprocess as claimed in claim 8; wherein the wetted granular pumice isdried at a temperature of 0°-200° C.
 13. The process as claimed in claim8; wherein the hydrophobicizing agent comprises at least one silane, atleast one siloxane, or a mixture thereof.
 14. The process as claimed inclaim 13; wherein the silane, if utilized as the hydrophobicizing agent,is: A1) silane of the general formula:R³—Si—(OR²)₃; where: R² is a methyl or ethyl radical; and R³ is:  analkyl radical having from 3 to 12 carbon atoms; or  an aminoalkylradical having from 1 to 6 carbon atoms; or  a radical of the generalformulaH₂N—(CH₂)_(x)R⁴—(CH₂)_(y)—, where:  R⁴ is an oxygen, sulfur, —NH—, or —NH—CH₂—CH₂—NH— radical; and  x≧2; and y≧2; or  a radical of thegeneral formula:(R²O)₃Si—(CH₂)_(x)—NH—(CH₂)_(x)—,  where R² and x are as defined above;

 where n is an integer from 1 to 5; wherein the siloxane, if utilized asthe hydrophobicizing agent, is: A2) a polysiloxane or siloxanol of thegeneral formula:

where: R⁵ is a methyl or phenyl radical, with at least 90% of theradicals R⁵ being methyl radicals; and m=20 to 250; and wherein themixture, if utilized as the hydrophobicizing agent, comprises: at leastone siline as per A1); and at least one siloxane as per A2).
 15. Theprocess as claimed in claim 8; wherein the hydrophobicizing agentcomprises at least one compound selected from organic fatty acids andsalts of organic fatty acids.
 16. The process as claimed in claim 8;wherein the hydrophobicizing agent comprises mineral oil.
 17. Theprocess as claimed in claim 8; wherein the hydrophobicizing agentcomprises an alkane.
 18. The process as claimed in claim 8; wherein thehydrophobicizing agent comprises bitumen.
 19. The process as claimed inclaim 8; wherein the outer phase of the emulsion comprises water. 20.The process as claimed in claim 8; wherein a content in grams of thehydrophobicizing agent in the wetted granular pumice is 0.1% to 2.5% ofan amount in grams of granular pumice which is wetted.
 21. The processas claimed in claim 8; wherein an amount in grams of the outer phase ofthe emulsion is ⅕ to ⅓ of an amount in grams of granular pumice which iswetted.